The transportation and distribution of bulk materials is an essential component of the economy of industrialized nations. Because a considerable percentage of the transportation and distribution of bulk materials is accomplished by trucks, rail cars, and sea-going vessels, highly efficient and cost-effective systems for the unloading of such vehicles represent an important link in this economic chain. This is especially true in the case of sea-going vessels that are off-loaded by transfer of materials first to much smaller, shallow drafted lighters or barges, and then into shore-based storage or transportation facilities.
Although there are many systems for the handling of bulk materials in general, a predominant system used for unloading comprises a chain or belt bucket elevator operated from a single digging boom. The boom is pivotably attached to an overhead support positioned above the material to be unloaded or moved. Elevation of the boom with respect to the load is controlled by winches. In the case of, say, barges, the bucket angle of attack relative to the barge is fixed by a short (less than 10 ft.) catenary formed at the digging end of the boom. The usual four-sided buckets are emptied by centrifugal force and/or gravity at the discharge end of the boom onto a conveyor.
While this system of barge unloading has been operational for many years, it presents several disadvantages which reduce its mechanical efficiency and cost-effectiveness. First, elevation of a single digging boom by a winch or hoist system can only be accomplished by pivoting about a restricted angle at the coupling between an overhead support and the digging boom. This restriction, combined with the fixed, short catenary, produces a substantially invariant angle of attack at the bucket-material interface. Second, a fixed catenary is unable to compensate for, say, wave action or other changes in load elevation, resulting in a loss of efficiency under normal operating conditions. Third, the limited drag-line action created by buckets in a short catenary diminishes the potential efficiency of the system. Fourth, centrifugal and or gravity discharge of the four-sided bucket requires a 180.degree. rotation of each bucket during emptying. In addition to creating an environmental hazard from dust spillage and carry back of material, centrifugal and/or gravity discharge necessitates extension of the discharge end of the digging boom far above the transport conveyor and the angle change of the digging boom changes the transfer position relative to the conveyor. This design feature increases the horsepower requirements of the drive system. Moreover, the large capacity buckets currently used limit the chain or belt speed and require further increases of drive system horsepower to maintain unloading capacity.